Typical petroleum drilling operations employ a number of techniques to gather information about the borehole and the formation through which it is drilled. Such techniques are commonly referred to in the art as measurement while drilling (MWD) and logging while drilling (LWD). As used in the art, there is not always a clear distinction between the terms LWD and MWD. Generally speaking MWD typically refers to measurements taken for the purpose of drilling the well (e.g., navigation) and often includes information about the size, shape, and direction of the borehole. LWD typically refers to measurement taken for the purpose of analysis of the formation and surrounding borehole conditions and often includes various formation properties, such as acoustic velocity, density, and resistivity. It will be understood that the present invention is relevant to both MWD and LWD operations. As such they will be referred to commonly herein as “MWD/LWD.”
Transmission of data from a downhole tool to the surface is a difficulty common to MWD/LWD operations. Mud pulse telemetry is one technique that is commonly utilized for such data transmissions. During a typical drilling operation, drilling fluid (commonly referred to as “mud” in the art) is pumped downward through the drill pipe, MWD/LWD tools, and the bottom hole assembly (BHA) where it emerges at or near the drill bit at the bottom of the borehole. The mud serves several purposes, including cooling and lubricating the drill bit, clearing cuttings away from the drill bit and transporting them to the surface, and stabilizing and sealing the formation(s) through which the borehole traverses. In a typical mud pulse telemetry operation, a transmission device, such as an electromechanical pulser or a mud siren located near the drill bit generates a series of pressure pulses (in which the data is encoded) that is transmitted through the mud column to the surface. At the surface, one or more transducers convert the pressure pulses to electrical signals, which are then transmitted to a signal processor. The signal processor then decodes the signals to provide the transmitted data to the drilling operator.
One common problem with decoding a mud pulse signal is that the signal to noise ratio is often low owing both to low signal amplitude and high noise content. The amplitude of a transmitted pressure pulse tends to attenuate as it travels up the drill pipe. Such attenuation is dependent on many factors including the depth of the borehole, the type of drilling mud, the hydrostatic pressure, the number of joints in the drill string, and the width of the pressure pulse. Moreover, there are a number of potential sources of noise generated during drilling operations including turning of the drill bit and/or drill pipe in the borehole, sliding and/or impact of the drill pipe against the borehole wall, and the mud pump that is used to pump the mud downhole. Another source of noise is created by reflected signals that are generated when the original pressure pulse hits a pulsation dampener (also referred to in the art as a desurger) near the top of the mud column and reflects back downhole.
To obtain reliable MWD/LWD signal decoding, slow data transmission rates (e.g., on the order of about 1 bit per second) are typically used in order to achieve an acceptable signal to noise ratio. When data transmission rates are increased, the signal to noise ratio tends to decrease due to decreased signal amplitude, thereby decreasing the reliability of the transmitted data. In a typical drilling application, the narrowest pulse that can be properly decoded is about 0.4 seconds or greater. Pressure pulses less than about 0.4 seconds tend to be lost in the background noise.
Recently, techniques employing a high-resolution transducer or two longitudinally spaced transducers have been developed to reduce the effects of noise (and therefore to increase the signal to noise ratio). In the dual transducer configuration, the signal at a second transducer is subtracted from the signal at a first transducer. Various electronic filters are also typically used in such applications. One such technique (disclosed in U.S. Pat. No. 6,308,562 to Abdallah et al.) utilizes a first transducer on the standpipe and a second at or near the pulsation dampener. The technique further utilizes an adaptive noise canceller to produce a processed signal with more sharply defined leading and trailing edges. A high-resolution transducer provides some improvement over traditional single transducer systems, but the signal to noise ratio can be unacceptably high even with such improved transducers.
Therefore, there exists a need for an improved drilling fluid pressure pulse detection apparatus and methods for detecting transmitted pressure pulses in the drilling fluid. In particular, there exists a need for a apparatus capable of detecting high speed, low amplitude pressure pulses.